An airline produces its transport service on its flight network, just like a factory's production line. So if there are not enough passengers to open a direct flight from Trabzon to Antalya, how do those passengers reach Antalya? In this lesson we examine, step by step, what a flight network is, the spatial and timing approaches in network design, how network structures are classified, and the hub-and-spoke system, which creates many connections from a small number of airports, together with waves, the connection window and the choice of hub airport.

What will you learn in this episode?

• To define the airline flight network, explain the size and breadth of a network and explain how network structure affects the schedule-based components of the product
• To compare the spatial approach and the timing approach in network design
• To classify network structures by their concentration in time and space
• To explain the hub-and-spoke network structure with the concepts of hub airport, spoke airport, wave and connection window
• To explain the types of hub-and-spoke networks and the connection-focused and productivity-focused approaches in setting the network and schedule
• To explain the criteria for choosing a hub airport, the difference between a base airport and a hub airport, and path dependence

Summary of this episode

1. What is a flight network?

Definition (network): In its most general definition, a network is a structure made of points called nodes and the lines connecting these points. The streets of a city, power lines or water channels are well-known examples of networks; Graph Theory is used to analyze networks.

Definition (airline flight network): The whole set of flight routes connecting flight points in time and space.

Why is it a set of products? Because transport between each airport pair is a separate product, the flight network can also be seen as the union of the airline products to be produced.

Size and breadth: The size of a network is measured by the number of cities or airport pairs served, and its breadth by the geographic area over which these points are spread.

2. Network structure and schedule-based components

Explanation: Network structure directly affects the schedule-based components of the product, which passengers see as an important demand determinant.

Total travel time: First among these components is the total travel time between the true origin and the true destination of the journey.

Other components:

• flight frequency,
• departure and arrival times,
• the size and breadth of the network,
• whether the flight is direct or connecting,
• on-time performance.

Definition (on-time performance): It shows, as a percentage, how many of every 100 flights depart within the first 15 minutes of the scheduled time.

3. The spatial approach and the timing approach

Definition (spatial approach): The approach that prioritizes geographic variables in network design. It asks: Where will the connection points be, which points will be connected directly, how many indirect connections will be obtained via a point, on which routes will the flow concentrate? The location of the collection center or of the airport to be used as a base is determined with this approach. Designs that prioritize space mostly focus on reducing costs.

Definition (timing approach): The approach that prioritizes at what times of day the points will be connected and how this affects total travel time. It mostly puts demand and revenue at the center.

Why does time matter too? Offering connections to many points is valuable for the passenger; however, waiting long at a connection lengthens total time and reduces the time benefit the airline creates.

Example: If a geographically ideal airport has a capacity limit, an effective wave system may not be set up; a point where night flights are restricted because of noise cannot be assessed only in terms of space either.

Comparison (two approaches): The spatial approach asks "where?" and focuses on cost; the timing approach asks "when?" and focuses on demand and revenue. An effective design uses the two approaches together, but classifications mostly use spatial variables.

4. Classification of network structures

Criterion: The shape of networks is determined by where and when traffic flow concentrates.

Linear network structure: It appears if points with heavy flows form a line in space when connected.

Hub-and-spoke network structure: It forms if the flow concentrates at a single point and in deliberately set time windows of the day.

Grid network structure (grid network): If there is no effort to concentrate in time and space, and all points are connected directly to each other with point-to-point flights, this grid-like structure is in question; the flights in this network are also called cross flights. Some authors use point-to-point network and grid network in the same sense.

In practice: Airlines often use derived networks that resemble these structures.

5. The hub-and-spoke network structure

Definition: A network structure in which flights departing from spoke airports land at a geographically central airport within a given time window and take off again towards the spoke points within about 1–2 hours. At the hub, passengers are grouped according to the final destination they actually want to reach and transferred to the relevant flights.

Where does the name come from? The English name comes from the analogy of a cartwheel: the hub represents the hub airport, and the spokes reaching out to the rim represent the routes to the spoke airports. The Turkish equivalent "topla-dağıt" (collect-distribute) has become widely accepted because it highlights the structure's function.

Definition (hub airport): An airport that serves a transfer function, where a wave structure designed with time and space in mind is used to increase indirect connections and reduce transfer time.

Definition (wave): The concentration of flights arriving at and departing from the hub airport within a given time window. The arrival wave brings passengers to the shore, as it were, and the departure wave carries them out to sea a while later.

Definition (connection window): The time window between the arrival and departure waves in which passengers and baggage are transferred. It must be wide enough for the passenger to be transferred with their baggage, and narrow enough to avoid unnecessary waiting.

Definition (minimum connection time; MCT): The shortest time needed for passengers, baggage and cargo to be transferred from one flight to another. The connection window cannot be shorter than the MCT.

Important rule: Concentration that happens by chance does not make a network hub-and-spoke; flights must be deliberately synchronized.

Example (fictional): Let an airline choose the airport in Ankara as its hub, with ten spoke airports directly connected to it. There are passengers who want to go to Antalya from Trabzon, Sivas, Kars, Erzurum, Van, İzmir, Diyarbakır, Gaziantep and Şanlıurfa, but none of these cities has enough demand to open a direct flight to Antalya. The aircraft departing from these cities at 09.00 in the morning meet in Ankara at 10.30, passengers are transferred, and at 12.00 the Antalya flight takes off with a sufficient load factor. If a similar wave is set up in the evening, the airline will have operated a hub-and-spoke network with two waves a day.

6. Types of hub-and-spoke networks

First criterion (the hub's own demand): Hubs with a wide and populous catchment area set up networks with high direct-flight potential.

Definition (wayport hub): The network formed by hubs that benefit from transfer traffic thanks to their unique geographic location, even if their direct demand is limited. The source gives Türkiye's location in the middle of global traffic flows as an example of this.

Second criterion (flight length in connections): The distance of the connected regions from the hub is considered.

Definition (hourglass network): A network that connects distant regions, in which arrival and departure waves are in opposite directions; this prevents unnecessary backtracking. It has one-way and multi-directional forms.

Definition (hinterland hub): A network that connects distant regions with regions close to the hub, that is, connects long-haul routes to short-haul routes via the hub.

Third criterion: The degree to which the hinterland network focuses on a particular region.

Definition (global flight network): A network spread across the world, formed by combining several hub-and-spoke networks.

7. The connection-focused and productivity-focused approaches

What increases the number of connections? The main factor is the number of flights in a wave. Example: 10 arrival and 10 departure flights create 10 × 10 = 100 connections.

Effect of the connection window: As the window widens, the chance of making a connection increases, but waiting and total travel time get longer and the perceived quality of the connection falls.

Effect of wave directions: More wave directions can reduce the number of connections. Example: In the source's example, when an opposite direction is added to the east-west wave and the flights are split in two, the number of connections falls from 100 to 50.

Where should aircraft stay overnight? Aircraft staying overnight at spoke airports can fit one more wave into the day; on the other hand, since maintenance facilities and flight crews are usually at the hub, this choice creates cost.

The order of long-haul flights: When transferring from international to domestic flights, MCT is longer because of passport and customs procedures; that is why long-haul flights come first in the collection wave and last in the distribution wave.

Definition (productivity-focused approach): The approach that puts the productivity of aircraft and pilots first. Aircraft productivity is measured by the average daily utilization of the aircraft in the fleet; as the ratio of productive time in the air to time on the ground increases, productivity increases.

The cost of wide windows: Wide windows keep aircraft on the ground, lower labor productivity and increase airport charges.

Why are buffer times added? If waves are set very close to turnaround time and MCT, short delays have a knock-on effect; that is why buffer times are placed between waves.

Comparison (two approaches): The connection-focused approach prioritizes the number and quality of connections offered to passengers, while the productivity-focused approach prioritizes the productive use of aircraft and staff. The basic measure of success is increasing the number of value-creating connections without compromising connection quality and productivity.

8. Choosing the hub airport

Two groups of criteria: features that increase the number and quality of connections, and features that increase productivity.

Location: Being in the middle of the traffic flow is like opening a shop where many people pass by.

Other features assessed: the capacity of the runway, taxiway and apron areas, the terminal and the airspace at busy hours, rapid exit taxiways, weather conditions, the efficiency of ground handling, security and baggage systems, a single terminal and enough jet bridges, operating hours, charges and room for growth.

Definition (base airport; base): The airport an airline uses mainly for reasons such as maintenance, crew location and operations.

Comparison (base and hub): Traffic can concentrate at a base airport; but if this concentration is not the result of deliberate wave design, the structure does not count as hub-and-spoke and that airport is not a hub airport.

Definition (path dependence): Past decisions and investments shaping today's choices. Investments made in an airport in the past, rights and slots gained, and crews settled there can make moving to another hub very costly.

In short

The flight network is the airline's production area and determines the schedule-based components of the product. The hub-and-spoke structure deliberately concentrates traffic at a hub airport in both space and time; waves and the connection window are the heart of this system. A good design strikes a balance between the number of connections, connection quality and aircraft productivity, and does so on a well-chosen hub airport.

Key concepts

• Airline flight network: The whole set of flight routes connecting flight points in time and space.
• Size and breadth of a network: Size shows the number of airport pairs served, breadth the geographic area over which these points are spread.
• On-time performance: The percentage of every 100 flights that depart within the first 15 minutes of the scheduled time.
• Spatial approach: The approach that prioritizes geographic variables in network design and mostly focuses on reducing costs.
• Timing approach: The approach that prioritizes at what times of day flights are operated and transfer times, mostly putting demand and revenue at the center.
• Hub-and-spoke network structure: A network structure that gathers passengers from spoke airports at the hub airport and redistributes them according to their final destinations.
• Grid network structure: A network in which all flight points are connected directly to each other and flights are point-to-point and direct.
• Hub airport: An airport that serves a transfer function, where a wave structure is used to increase indirect connections and reduce transfer time.
• Wave: The concentration of flights arriving at and departing from the hub airport within a given time window; it is divided into an arrival wave and a departure wave.
• Connection window: The time window between the arrival and departure waves in which passengers and baggage are transferred.
• Minimum connection time (MCT): The shortest time needed for passengers, baggage and cargo to be transferred from one flight to another.
• Wayport hub: A hub-and-spoke network set up to benefit from transfer traffic thanks to the hub's unique geographic location.
• Hourglass network: A hub-and-spoke network that connects distant regions, in which arrival and departure waves are in opposite directions.
• Hinterland hub: A hub-and-spoke network that connects long-haul routes to short-haul routes via the hub.
• Base airport (base): The airport an airline uses mainly for reasons such as maintenance, crew and operations; without deliberate wave design it does not count as a hub airport.
• Path dependence: Past decisions and investments shaping today's choices.

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Sources used:
Gerede, E. (2016). Havayolu işletmelerinin ağ yapıları. E. Gerede ve A. E. Demirci (Ed.), Havayolu Yönetimi (5. Ünite) içinde. Eskişehir: Anadolu Üniversitesi Yayınları.

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